1. Field of the Invention
There is provided a process for producing a styrene based polymer and a molded article comprising the polymer.
2. Description of the Related Art
Styrene based polymers have rigidity, excellent dimensional stability and are low-priced. As a result, they have been widely used for molded articles. It is further noted that there has been a recent demand for making lighter, large-sized moldings in injection molding applications. For making lighter, thinner walled, molded articles, it is essential to have high flowability during molding in addition to high mechanical strength.
In an attempt to meet such requirements, a method has been proposed in which the molecular weight of the polymer is increased in order to obtain a polymer with higher mechanical strength. This method is, however, disadvantageous in that short shot tends to occur on molding due to lowered flowability of the polymer. Even if an article is obtained by the method, the method is disadvantageous in that residual strain of the molded article is increased due to its higher flow shearing, resulting in lowered mechanical strength.
A method in which a plasticizer, such as mineral oil, is added to a polymer has been proposed in order to increase the flowability of the polymer without lowering its molecular weight. The method is disadvantageous, however, in that the heat resistance and the mechanical strength of the polymer are decreased due to the added plasticizer.
In accordance with the disclosure in Flory et al. (L. Am. Chem. Soc., 67, 2048, (1945)), i.e., that mechanical strength of a polymer is proportional to the reciprocal of its number average molecular weight (Mn), it is necessary to increase the number average molecular weight (Mn) of the polymer in order to increase the mechanical strength of the polymer. Also, it is necessary to lower the weight average molecular weight (Mw) of the polymer in order to increase its flowability. Further, Mn should be less than or equal to Mw, it is desirable for a polymer to have narrower molecular weight distribution (Mw/Mn) in order to have high flowability and high mechanical strength.
In order to narrow the molecular weight distribution, a polymerization method using anion polymerization has been proposed. Anion polymerization is disadvantageous, however, in that it requires severe reaction conditions and the manufacturing costs associated therewith are high, as compared with free radical polymerization. Moreover, the product may be colored by residual catalyst.
U.S. Pat. No. 5,401,804 discloses a free radical polymerization method for obtaining a thermoplastic resin having a narrow molecular weight distribution by mixing a free-radical initiator and stable free-radical agent with a polymerizable monomer compound and maintaining the mixture at a temperature of 100.degree. to 160.degree. C. The number average molecular weight of the resin obtained by this method is at most 58,000. Thus, the mechanical strength of the resin is insufficient for use in injection molding, extrusion molding and foam molding.